1. Field of the Invention
The present invention generally relates to image forming devices and, more particularly, relates to an image forming device which prints out information on a recording sheet by making a latent image on a recording drum.
In recent years, electrophotographic printers of a serial type have been developed, which type carries an electrophotographic process unit on a carriage for printing information. This development was made in order to meet a demand for miniaturization and lower pricing of electrophotographic recording devices. These printers of the serial type duplicate an image onto a recording sheet using an image-transfer unit by running the carriage in a direction perpendicular to a direction in which the recording sheet is fed, or advanced. However, the recently developed such printers require further refinement and improvement in printed-image qualities.
2. Description of the Prior Art
FIG. 1 shows a cross-sectional view of an electrophotographic recording device 10 of the prior art and which includes a photosensitive drum 11, an electrification unit 12, an optical scanning unit 13, a developing unit 14, an image-transfer unit 15, a discharging unit 16, a cleaner 17, an image-fixing unit 18, and sheet conveying units 19a and (which comprise drive rollers). Also, the electrophotographic recording device 10 is provided at one side thereof with a predetermined number (two in this example) of sheet supplying units 21a and 21b storing recording sheets 20, and at the other side with a stacker 22 which stacks the recording sheets 20 into a pile, or stack.
The photosensitive drum 11 is at least as wide as the recording sheets 20, and has an axis of rotation 11a, which is perpendicular to a direction in which the recording sheets 20 are fed, or advanced. The developing unit 14 is filled with toner 14a, and has a developing roller 14b, which touches (i.e., contacts) the photosensitive drum 11 to form a toner image thereon. The photographic image-fixing unit 18 includes fixing rollers 18a and 18b with a heat source 23 being provided inside the fixing roller 18b.
FIG. 2 shows an isometric view of an optical system employed in the electrophotographic recording device 10 of FIG. 1. In FIG. 2, the optical system includes a semiconductor laser 24, a collimator lens 25, a diaphragm 26, a cylindrical lens 27, a polygon mirror 28, a toric lens 29, and an f-.THETA. lens 30. A laser light beam emitted by the semiconductor laser 24 is reflected by the polygon mirror 28 after passing through the collimator lens 25, the diaphragm 26, and the cylindrical lens 27, so as to be scanned on the photosensitive drum 11. The light beam reflected by the polygon mirror passes through the toric lens 29 and the f-.THETA. lens 30 before reaching the photosensitive drum 11.
When there is a dispersion error on a reflectance surface of the polygon mirror 28, timing variations are created in writing information by scanning a laser beam on the photosensitive drum 11. As a counter-measure for this, at a beginning of each scan corresponding to each reflectance surface, a timing detection sensor 33 receives the laser beam, as reflected by a fixed mirror 31 via a beam-condensing lens 32. Also, when there is an error in the positioning of the polygon mirror 28 with regard to an angle of its rotation axis, a variation is created in the location on the photosensitive drum 11 of the laser beam. The toric lens 29, which is asymmetric with regard to its rotation, is used for correcting the error.
In the electrophotographic recording device 10 of FIG. 1, the photosensitive drum 11 is electrified by the electrification unit 12, and is illuminated by the laser beam coming from the optical scanning unit 13. In this manner, a latent image is recorded on the photosensitive drum 11 as a pattern of different voltage levels. The developing unit 14 attaches the toner 14a on the photosensitive drum 11 holding the latent image, so that a visible toner image is created thereupon.
The conveying rollers 19a and 19b lead (i.e., advance) the recording sheet 20 into a gap between the photosensitive drum 11 and the image-transfer unit 15, whereby the recording sheet 20 comes in contact with the surface of the photosensitive drum 11. The toner image on the photosensitive drum 11 is transferred onto the recording sheet 20 by the image-transfer unit 15. Then, the recording sheet 20 with the toner image thereupon is clamped by the fixing roller 18a and the fixing roller 18b, as heated to a predetermined temperature by the heat source 23, so that the toner image is fixed on the recording sheet 20.
FIG. 3 shows an isometric view of another optical system. In FIG. 3, an LED (light-emitting diode) array 34 comprised of LEDs arranged in a line is used as a light-exposure device. The LED array 34 is supplied with on/off signals for a scanning purpose, whereby the LEDs which are supplied with an on-signal emit light. Thus, a parallel scan can be realized.
Light beams emitted by the LED array 34 are condensed by a SELFOC lens 35 so as to write information on the photosensitive drum 11. Here, each of the LEDs emitting light constitutes one pixel in an image pattern. This optical system does not include mechanical elements which might affect precision in positioning the light beam. However, in order to adjust a diameter of the condensed light beam and light energy applied on the photosensitive drum 11, a distance between emitting surfaces of the LED array 34 and points of writing information on the photosensitive drum 11 should be measured.
There is a type of the electrophotographic recording devices, which type can control a position of an LED head, such as the LED array and the SELFOC lens, in relation to the photosensitive drum. Thereby the focusing of the light beam can be adjusted. Such a device is disclosed in the Japanese Laid-Open Patent Application No. 62-147472.
Also, there is a serial type of the electrophotographic printers, which type carries an electrophotographic process on a carriage in order to print information by rotating a photosensitive drum in a direction along the width of the recording sheet.
FIGS. 4A and 4B show a plan view of part of a serial-type electrophotographic printer and a cross-sectional view of a carriage of FIG. 4A, respectively.
In FIGS. 4A and 4B, a serial-type electrophotographic printer 41 disclosed in Japanese Laid-Open Patent Application No. 61-152463 includes conveying rollers 43a and 43b, which convey a recording sheet 42. The serial-type electrophotographic printer 41 also includes a shaft 44 parallel to respective axes of the conveying rollers 43a and 43b, and a carriage 45, which is movable in a direction along the width of the recording sheet 42 (perpendicular to the direction of conveying the recording sheet 42) . The movement of the carriage 45 is guided by the shaft 44 and driven by a motor (not shown). The serial-type electrophotographic printer 41 also includes an image-fixing unit 46 fixed in a predetermined position, which has a width wider than that of the recording sheet 42. There is an image-transfer unit 47 provided beneath the recording sheet 42, as shown in FIG. 4B.
The carriage 45 includes an image-holding body 51, an electrification unit 52, a light-exposure unit (an LED array and a lens) 53, a developing unit 54, toner 55, and a developing roller 56. The image-holding body 51 rotates at a speed corresponding to movement of the carriage 45. A surface of the image-holding body 51 is electrified by the electrification unit 52, and the light-exposure unit 53 forms a latent image of static charge on that surface. The latent image of static charge is turned into a visible toner image by the developing roller 56 of the developing unit 54 attaching the toner 55 on the above-mentioned surface. The toner image on the surface of the image-holding body 51 is transferred to the recording sheet 42 by the image-transfer unit 47 opposing the image-holding body 51 through the recording sheet 42. The image transferred onto the recording sheet 42 is fixed by the image-fixing unit 46.
In the serial-type electrophotographic printer 41 as described above, the rotation rate of the image-holding body 51 must be in accordance with the moving speed of the carriage 45. Thus, the rotation rate can become quite large in order to realize a practical printing speed.
For example, assume that it takes 30 seconds (2 pages per minute) for printing out one page of the recording sheet 42 having an A4 size (similar size to the legal paper size). Also, assume that a diameter of the image-holding body 51 is as small as 24 mm because of the demand for the miniaturization of the device, and that one page includes 10 lines with intervals between characters in one line being 28 mm. Then, a time length which can be used for one line is 3 seconds (30 seconds divided by 10 lines). Further, assume the carriage 45 uses 2 seconds for printing characters and 1 second for returning to the start point of a next line. Then, if a recording area in one line is 210 mm, the rotation rate of the image-holding body 51 must be equivalent to a speed of 105 mm/sec.
For the printer to be improved, of course, higher speed printing should be realized.
In the serial-type electrophotographic printer, a returning operation at a time of moving from one line to the next line must be accurate. Thus, correction is carried out for the returning operation with regard to a primary scanning direction P and a secondary scanning direction S. Accuracy in the secondary scanning direction S is dictated by the accuracy of conveying the recording sheet. Thus, the correction with regard to the secondary scanning direction S can be easily realized by adjusting a number of steps for the rotation of the conveying motor.
FIGS. 5A and 5B show illustrative drawings for explaining a displacement in the primary direction P. As shown in FIGS. 5A and 5B, when the light-exposure unit 53 is mounted at an angle to the image-holding body 51, the bottom one of the dots arranged along the width of one line is displaced relative to the top one of the dots in the primary scanning direction P. Thus, a displacement X is generated between one line and the next line. For example, in order for the displacement X to seem to be within one-dot precision to human vision, the displacement X must be smaller than half a dot in effect. Thus, when a resolution is 200 dpi, the displacement X must be smaller than 0.0635 mm.
The displacement X is mainly generated by a variation between different dots in a light-emitting line of the light-exposure unit 53 and by a variation in the positioning of the light-exposure unit 53. These variations are difficult to adjust, thus creating a problem of a degraded printing quality caused by the displacement.
The electrification unit 52 and the developing roller 56 must be rotated at almost the same rate as that of the image-holding body 51. However, a high rotation rate of the electrification unit 52 and the developing roller 56 results in the toner 53 being not completely attached on the image-holding body 51. Thus, some of the toner 53 ends up flying around the image-holding body 51.
The flying toner can become attached to the light-exposure unit 53 so as to create under-exposure. The light-exposure unit 53 cannot be placed at a sufficient distance from the image-holding body 51 because of the demand for the miniaturization. Also, the printer cannot be thoroughly cleaned by users for removal of the toner attached on the light-exposure unit 53. This also adds to the problem of degraded printing quality.
Further, the image-holding body 51 is a photosensitive drum formed of an organic photoconductive (OPC) material or amorphous silicon. The use of the amorphous silicon is preferable to the OPC material because a photosensitive surface thereof which receives energy of the light exposure should drop in the voltage level thereon at a fast enough rate so as to be ready for the development of a latent image.
However, an OPC material is generally used for a photosensitive drum because of the demand for a lower price. Such a photosensitive drum is typically rotated at 50 mm/sec, such as in the printer of FIG. 1. This creates problems of a shorter life of the photosensitive drum, an insufficient density of blackness, coloring of a background, and a degraded printing quality.
Further, adjustment of the focusing of light-exposure units 34, 35, and 53 must be conducted in the printers of FIG. 1 and FIG. 4. However, the adjustment of the focusing in accordance with a method disclosed in the Japanese Laid-Open Patent No.62-147472 on the page No.3 and in the figure No.3 thereof requires a number of repeated adjustments based on repeated checks on printing qualities. Thus, this method is actually difficult to carry out, leading to a degraded printing quality.
Accordingly, there is a need in the field of electronic photographic printers for a serial-type electronic photographic printer which has an improved printing quality.